[PATCH v2 02/11] xfs: reserve v5 superblock read-only compat. feature bit for finobt

[Brian Foster <bfoster@redhat.com>]

Reserve a v5 read-only compatibility feature bit for the finobt and
create the xfs_sb_version_hasfinobt() helper to determine whether
an fs has the feature enabled.

The finobt does not change existing on-disk structures, but must
remain consistent with the ialloc btree. Modifications from older
kernels would violate that constrant. Therefore, we restrict older
kernels to read-only mounts of finobt-enabled filesystems.

Note that this does not yet enable the ability to rw mount a finobt
fs (by setting the feature bit in the XFS_SB_FEAT_RO_COMPAT_ALL
mask).

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/xfs/xfs_sb.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/xfs/xfs_sb.h b/fs/xfs/xfs_sb.h
index 35061d4..070a7f6 100644
--- a/fs/xfs/xfs_sb.h
+++ b/fs/xfs/xfs_sb.h
@@ -585,6 +585,7 @@ xfs_sb_has_compat_feature(
 	return (sbp->sb_features_compat & feature) != 0;
 }
 
+#define XFS_SB_FEAT_RO_COMPAT_FINOBT   (1 << 0)		/* free inode btree */
 #define XFS_SB_FEAT_RO_COMPAT_ALL 0
 #define XFS_SB_FEAT_RO_COMPAT_UNKNOWN	~XFS_SB_FEAT_RO_COMPAT_ALL
 static inline bool
@@ -639,6 +640,12 @@ static inline int xfs_sb_version_hasftype(struct xfs_sb *sbp)
 		 (sbp->sb_features2 & XFS_SB_VERSION2_FTYPE));
 }
 
+static inline int xfs_sb_version_hasfinobt(xfs_sb_t *sbp)
+{
+	return (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) &&
+		(sbp->sb_features_ro_compat & XFS_SB_FEAT_RO_COMPAT_FINOBT);
+}
+
 /*
  * end of superblock version macros
  */
-- 
1.8.1.4

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